STP Indirect Topology Change

[bogdan CCSI #35055]

Hello guys,

I have some questions regarding a particular STP topology. This is related to 802.1d protocol.

The topology is as bellow:

I have shutdown the port between R2 and R1, so temporarily R2 has lost its root port.

In this particular case, because of this link down R2 will try to send TCN to RB via root port, but this is dead, in the same time RB - R1 will send the TCN via interface connecting to R3.

After shutting down the port the wireshark capture looks like on R2 interface connectiong to R3:

Frame 42, the last BPDU received on the root port and starting with 43 is the BPDU generated by R2 after shutting down the root port (announcing itself as root).

Could you please make the point clear in which way R3 will announce R2 about an alternative path to root bridge and unblock it's blocked port toward R2 to send out the new BDPU informing R2 to reach the RB on this new path?

Thank you very much!

[Reza Sharifi]

Hi,

In your case, since R2 fails to receive BPDU from R1 for a max age time of 20 seconds (default) the port connecting to R3 goes from blocking to forwarding.

HTH

[Mohamed Sobair]

Hello,

When R2 announce itself as a root bridge to R3, R3 recieves this inferior BPDU and it already has Superior (Better BPDU) from R1 as the STP root bridge.

R3 then waits for the STP Max age timer of the BPDU to expire before sending any BPDU to R2. Once it expires, R3 moves its port from BlK state to Forwarding state going through STP 802.1d Port states (Blocking, Listening, Learning and Forwarding) which is the Forward delay timer.

When the Port of R3 goes to forwarding state, it forwards the BPDU to R2 informing it about the existence of the root bridge R1 and R2 makes the new port as an STP root port.

However, this process normally in STP 802.1d takes about 50sec, In order to have better and fast STP 802.1d, its recommended to use PVST+ feature (Backbone Fast).

The (Backbone Fast) feature reduces this time (50 sec) by (20 sec) making it 30sec overall for STP to converge. The backbone fast feature prevent R3 from waiting for the (Max age Timer) of the BPDU to expire, allows it to send Better BPDU to R2 as soon as it recieves the announcement from R2.

HTH

Mohamed

[bogdan CCSI #35055]

Hello,

Thank you all for your answer.

The problem is in this way, R2 is sending BPDU to R3 stating that he is the root bridge after I shutdown R2 root port. This BPDU is received by the R3 on the blocked port and since R3 has a better BPDU on his RP will ingore the BPDU received from R2. This is STP normal way of working.

The Max Age is activated only if R3 will not receive BPDUs on his root port but here we don't have this case. R3 is ok, it is receiving BPDU from R1 which is the root bridge.

Please correct me if I'm wrong.

On the other hand, when the root port of R2 goes down, it will try to send TCN via his root port toward root bridge, but the port is down, R2 cannot send TCN on that port.

Now, I think here is the point, R1 will sense also this link down and will send TCN on the port connected to R3 (this is the single port in the up up state on R1). When R3 receives the TCN will reply with TCN ACK.

Now, I'm not sure about the fact in which way R3 is opening the blocked port toward R2, based on the TCN received from RB? Please open my eyes

[Mohamed Sobair · Accepted solution]

You are wrong!

When you Shutdown the root port of R2 and it doesnt recieve BPDU from the root bridge on its root port, It believes its the root bridge Now because it has no BPDU (No Superior root bridge BPDU).

When R2 believes its the root bridge it sends a New BPDU telling R3 I am the New STP root bridge.

When R3 recieves it, Of Course R3 has better BPDU recieved from R1, it doesnt immediately Ignore it, it however waits for the (Max age) Timer to expire and transit its port from blocking to forwarding state and sends the Superior BPDU telling R2 there is a root bridge a live here and reachable.

When R2 recieve the New BPDU, it compares it with its own BPDU and believe the new port is the root port and there is a root bridge reachable Via R3.

So the Conclusion here, For STP normal operation the Root Bridge Must Continously Sends STP BPDU informing all bridges about the presence of the root. All STP designated Bridges waits for a (max age) timer for this BPDU to be recieved from the root on its root port. If it fails to recieve it, a new STP root has to be elected. This is the reason Why R3 waits for the (Max age) timer to expire to make sure about the presence of R1 (The root)

Regards,

Mohamed

[bogdan CCSI #35055]

I have tested again by modifying the STP max age from 20s to 6s and on capture I saw what you said above.

Thanks!